A great deal of research is currently underway to develop treatments and cures for viral infections in humans and in animals. Notably the incidence of AIDS and AIDS related complex (ARC) in humans is increasing at an alarming rate. The five year survival rate for those with AIDS is dispiriting and AIDS patients, whose immune systems have been seriously impaired by the infection, suffer from numerous opportunistic infections including Kaposi's sarcoma and Pneumocystis carninii pneumonia. No cure for AIDS is known and current treatments are largely without adequate proof of efficacy and have numerous untoward side effects. Fear of the disease has resulted in social ostracism of and discrimination against those having or suspected of having the disease.
Retroviruses are a class of ribonucleic acid (RNA) viruses that replicate by using reverse transcriptase to form a strand of complementary DNA (cDNA) from which a double stranded, proviral DNA is produced. This proviral DNA is then randomly incorporated into the chromosomal DNA of the host cell making possible viral replication by later translation of viral message from the integrated viral genome.
Many of the known retroviruses are oncogenic or tumor causing. Indeed, the first two human retroviruses discovered, denoted human T-cell leukemia viruses I and II or HTLV-I and II, were found to cause rare leukemias in humans after infection of T-lymphocytes. The third such human virus to be discovered, HTLV-III, now referred to as HIV, was found to cause cell death after infection of T-lymphocytes and has been identified as the causative agent of AIDS and ARC.
The envelope protein of HIV is a 160 kDa glycoprotein. The protein is cleaved by a protease to give a 120 kDa external protein, gp120 and a transmembrane glycoprotein, gp41. The gpl120 protein contains the amino acid sequence that recognizes the CD4 antigen on human T-helper (T4) cells.
One approach being explored is to prevent the binding of HIV to its target, the T4 cells in humans. These T4 cells have a specific region, a CD4 antigen, which interacts with gp120. If this interaction can be disrupted, the host cell infection can be inhibited.
Interference with the formation of the viral envelope glyoprotein could prevent the initial virus-host cell interaction or subsequent fusion or could prevent viral duplication by preventing the construction of the proper glycoprotein required for the completion of the viral membrane. It has been reported [See H. A. Blough et al., Biochem. Biophys. Res. Comm. 141(1), 33-38 (1986)] that the nonspecific glycosylation inhibitors 2-deoxy-D-glucose .beta.-hydroxy-norvaline inhibit expression of HIV glycoproteins and block the formation of syncytia. Viral multiplication of HIV-infected cells treated with these agents is stopped, presumably because of the unavailability of glycoprotein required for the viral membrane formation. In another report [W. McDowell et al. , Biochemistry 24(27), 8145-52 (1985)], the glycosylation inhibitor 2-deoxy-2-fluoro-D-mannose was found to inhibit antiviral activity against influenza infected cells by preventing the glycosylation of viral membrane protein. This report also studied the antiviral activity of 2-deoxyglucose and 2-deoxy-2-fluoroglucose and found that each inhibited viral protein glycosylation by a different mechanism. However, other known glycosylation inhibitors have been shown to have no antiviral activity. Thus the antiviral activity against viruses in general, and the viral activity specifically, of glycosylation inhibitors is quite unpredictable.
It has been disclosed in U.S. patent application Ser. No. 295,856, filed Jan. 11, 1989, now abandoned, that a purified form of heparin, a sulfated polysaccharide, binds through interactions to a viral protein which is responsible for cell recognition and provides limited inhibition of host cell infection. However, heparin causes some side effects, notably hemorrhage and increased clot formation time as well as thrombocytopenia. Use of heparin is contraindicated in patients who are actively bleeding, or have hemophilia, purpura, thrombocytopenia, intracranial hemorrhage, bacterial endocarditis, active tuberculosis, increased capillary permeability, ulcerative lesions of the gastrointestinal tract, severe hypertension, threatened abortion or visceral carcinoma. The contraindication for use by hemophiliacs is particularly of concern because many such individuals are now HIV positive.
It has long been recognized that certain synthetic, water-soluble polymers exhibit a broad spectrum of biological activity [R. M. Ottenbrite in "Biological Activities of Polymers", Amer. Chem. Soc. Symp. Ser. No. 182, pp. 205-220, eds. C. E. Carraher and C. G. Gebelein (1982)]. A copolymer of divinyl ether and maleic anhydride has been shown to be active against a number of viruses and its use in cancer chemotherapy has been studied for years [Breslow, D. S. Pure and Applied Chem. 46, 103 (1976)]. Polyacrylic, polymethacrylic and a variety of other aliphatic backbone water soluble polymers also have been shown to have a broad spectrum of biological activities [W. Regelson et al., Nature 186, 778 (1960)]. Unfortunately, the extreme toxicity of these polymers has prevented their clinical use. Also, these polymers have a high molecular weight and are unable to pass through the renal membranes.
Attempts have been made to circumvent the toxicity and excretion problems by synthesis of low molecular weight (1,000 to 10,000) aliphatic polymers [R. M. Ottenbrite in "Biological Activities of Polymers", Amer. Chem. Soc. Symp. Ser. No. 182, pp. 205-220, eds. C. E. Carraher and C. G. Gebelein (1982)]. It has been found that such polymers are less toxic but have much reduced antiviral activity. These low molecular weight aliphatic polymers may be classed as "random coil" polymers. Such polymers have an unpredictable configuration because of the flexibility of the backbone linking groups. The configuration of random coil polymers in solution may be generally described as globular. Although the mechanism of action of such water-soluble polymers is unknown, one postulate is that the polymer binds to the viral membrane, e.g. encephelomyocarditis, through an ionic attraction, thus rendering the virus unable to infect host cells.
An additional synthetic polymer approach is to place ionic groups on the backbone of a polymer which exhibits a more defined geometry. There are numerous examples of non-ionic, synthetic polymers which exhibit a more linear geometry in non-aqueous solution than do the aliphatic polymers described above [J. Macromolecular Sci-Reviews in Macromol. Chem. Phys. C26(4), 551 (1986)]. The factors involved which cause this non-random coil structure are complex and poorly understood. In general, such polymers have either a very limited number of rotatable bonds which are not parallel to the polymer axis, or there is hydrogen bonding or dipolar interactions which favor linear structures. These polymers are referred to as having a "rigid backbone". A polyamide derived from terephthalic acid and p-diaminobenzene (known commercially as Kevlar.TM. supplied by DuPont) is a well-known example of such polymers.
Synthetic, water-soluble, rigid polymers are much less common, but a few high molecular weight examples are known (e.g. see U.S. Pat. No. 4,824,916 and 4,895,660). The non-random coil structure of this class of polymer results in high solution viscosities for a given molecular weight and concentration.
Clearly, it would be desirable to find a treatment and cure for AIDS and ARC which would display minimal or no side effects and constitute a clear improvement over the polymers previously employed as a pharmaceutical.